The invention relates to a combination drill bit which is designed to drill holes by annular cutting and continuous core breaking.
The new combination drill bit is designed to carry out a process for drilling by annular cutting and continuous core breaking.
Experiments were carried out with jet beam cutting a core by annular cutting, which core is broken by a rock bit, c.f. Maurer, W. C. Heilhecker, J. K. and Love, W. W., "High Pressure Drilling"--Journal of Petroleum Technology, July 1973. These experiments resulted in an increase of the drilling rate by 2-3 times. The problem in utilizing a jet beam is that it requires a down-hole pumpe, which is able to produce the very high pressure necessary to enable the liquid beam to cut the formation.
Previously, PDC (polycrystalline diamond compact) cutting elements and rock bits with teeth were combined, but then mainly with the intention to limit drilling advancement in soft formations in order to avoid clogging of the cutting elements, cf. U.S. Pat. No. 4,006,788.
At present, mainly two kinds of drill bits are used, i.e. PDC drill bits and rock bits. PDC drill bits cut the formation with the aid of an edge comprised of a number of PDC cutting elements. Due to the fact that the cutting elements rotate at the same rotational speed about a common axis, cutting speed will vary from zero at the center, to a maximum outermost on the periphery of the drill bit. It is, thus, impossible to achieve an optimal cutting speed of all cutting elements at the same time.
The cuttings formed when PDC cutting elements are used, often are very small, resulting in the fact that very limited geological information can be extracted from them. PDC-bits were constructed which cut a small core for use in geological analysis, cf. U.S. Pat. No. 4,440,247. Drilling operators reported that their effect as regards acquiring larger pieces is quite low.
The edge of a present PDC cutting element is 90.degree. and sharp. Consequently, it is comparatively weak and tends to chip.
Rock bits break up the formation, by teeth which are mounted on the rock bits being urged towards the formation by so high a force that the formation will break under and around said teeth. Due to the relatively plane face of the hole bottom, crack propagation due to each tooth penetration is of relatively small effect as regards the volume to be drilled. If the volume to be broken is acquired in the shape of an unstabilized core, the efficiency of each tooth penetration will be considerably improved.
Conventionally, the principle of annular cutting with continuous core breaking is not used, at present, for drilling holes. There are a number of patents based on this principle. According to one patent, diamonds baked into a matrix are used. This system provides for more grinding than cutting, requiring high rpm to achieve a satisfactory drilling advancement. The central rolling cones, which are used to break the core, then have to be run at too high rpm, cf. U.S. Pat. No. 3,055,443. According to another patent, edges of tungsten carbide are used, resulting in a very limited life of the drill bit due to insufficient resistance to abrasion of the edges. The last mentioned drill bit does not generate a cavity about the core before it is broken, i.e. the internal wall of the core drill bit has a stabilizing effect on the core, cf. U.S. Pat. No. 3,075,592. A third patent discloses utilizing cutting edges requiring channels/grooves in front of/behind the edges. The channels/grooves must be large enough to permit the pieces of broken core to pass to the outside of the drill bit. The core is broken by the aid of a toothed roller which has too much scraping effect due to its geometry. This will cause the teeth of the roller to be worn down far to rapidly. Nozzles are used to flush the toothed roller and to moisten the core so as to weaken it, cf. U.S. Pat. No. 2,034,073.
U.S. Pat. No. 5,016,718 discloses a combination drill bit for continuous drilling an annular, downwardly deepening hole coaxially surrounding an upwardly projecting cylindrical core, and progressively crushing axially successive increments of the core from the upper end of the core. This known drill bit has a drill bit body with a downwardly opening internal undercut cavity, a plurality of downwardly acting cutting elements on the lower end of the drill bit body, around the cavity opening, as well as a core crushing tool disposed in the cavity, the effective diameter of the core cruching tool being greater than that of the cavity opening. Whereas this known combination drill bit displays excellent drilling performance, some problems have been observed. During drilling in plastic formations one has observed that the core crushing tool has a tendency to compress the core with the result that the upper part of the core wall will go against the cavity inner wall, thus regaining sideways support. This effect may reduce the effect of the undercuting considerably. A second observation is that the stationary abrasive core formation is pressed against the rotating cavity inner wall, thereby wearing down the inner wall, even to the possible extent that the drill bit body is cut through, resulting in a drill bit wrecking with a consequential necessity of having to fish out the separated body piece from the drill hole.
Another effect has been observed at the axially downwardly facing lower end where a plurality of downwardly acting cutting elements are mounted and where internal drilling fluid delivery channels open. This observed effect is a certain washing out of drilled out material behind the cutting elements whereby the back support for these cutting elements is reduced. This washing out for drilled out material is assumed due to the formation of fluid turbulence in the area behind the cutting elements.
It is therefore an object of the present invention to provide a combination drill bit of the general type as disclosed in said U.S. Pat. No. 5,016,718, whereby the above mentioned problems are eliminated.